1. Field of Technology
This invention relates to a device for obtaining a synthetic composite video signal to reproduce a so-called picture-in-picture being inserted into a main picture on a screen of a display device such as a cathode ray tube, using one of the plural composite video signals entered simultaneously as a main picture and the remaining as reduced sub-picture(s).
2. Description of the Prior Art
FIG. 5 is a block diagram of a conventional video signal synthesizer, in which there is only one sub-picture.
In FIG. 5, reference numeral (11) represents a first input terminal in which a first composite video signal for a main picture (hereinafter called main picture signal) (1) is entered: (12) is a first YC separator; (13) is a first chroma demodulator; (14) is a first sync signal separator; (15) is a second input terminal in which a second composite video signal for a sub-picture (hereinafter called sub-picture signal (2) is entered. The main picture is reproduced on a full screen of a display device and the sub-picture is reproduced within the framework of, and in superimposed relationship with, the main picture on the same screen. The other components consist of the following: (17) is a second chroma demodulator; (18) is a second sync signal separator; (19), (20), (21) are first, second, third AD converters; (22), (23); (24) are first, second, third digital memories capable of writing and reading simultaneously and in asynchronous timing mutually; (25), (26), (27) are first, second, third DA converters; (28) is an analog switch; (29) is a chroma modulator; (30) is a YC adder, (31) is an output end; and (32), (33) are first and second digital signal processing system controllers.
Referring now to operation, the main picture signal (1) entering from the first input terminal (11) is separated into a first luminance signal (34) and a first color subcarrier signal (35) by means of the first YC separator (12). The first color subcarrier signal (35) is then demodulated into a first red color difference signal (36) and a first blue color difference signal (37) by the first chroma demodulator (13).
Further, the sub-picture signal (2) entering from the second input terminal (15) is separated into a second luminance signal (38) and a second color subcarrier signal (39) by the second YC separator (16). The second color subcarrier signal (39) is then demodulated into a second red color difference signal (40) and a second blue color difference signal (41) by the second chroma demodulator (17).
The first, second, third AD converters (19), (20), (21) convert the second luminance signal (38), second red color difference signal (40) and second blue color difference signal (41) into digital signals respectively. These digital signals are then written into the first, second, third digital memories (22), (23), (24). At this time, first and second control signals (42), (43), necessary for writing control are generated by the first digital signal processing system controller (32). Their timings are then synchronized with the sync signal (44) separated by the second sync signal separator (18), that is, the sub-picture signal (2).
Next, when reading out the contents from the first, second and third digital memories (22), (23), (24), the sub-picture is reduced, that is, compressed in the time direction. The compression in the time direction is realized by scanning, when reading out, the contents in the digital memories (22), (23), (24) at a speed of K times (K is greater than 1) the writing-in speed. As a result digital signals compressed to 1/K in the time direction are obtained. These time-compressed digital signals are converted into analog signals by the first, second and third DA converters (25), (26), (27). Through this digital signal processing system, the second luminance signal (38), the second red color difference signal (40) and the second blue color difference signal (41) are respectively time-compressed to become a third luminance signal (45), a third red color difference signal (46) and a third blue color difference signal (47), respectively.
Here, third and fourth control signal (48), (49) necessary for reading out, are generated by the second digital signal processing system controller (33). Their timings are then synchronized with the sync signal (50) separated by the first sync signal separator (14), that is, the main picture signal (1), and the obtained third luminance signal (45), the third red color difference signal (46) and the third blue color difference signal (47) are, as a matter of fact, synthesized with the main picture signal (1).
The analog switch (28), with three switching circuit systems for luminance signal and two color difference signals, is used in the time-division multiplexing of the main picture signal and the time-compressed sub-picture signal. The changeover signal (51) of this analog switch (28) is given in the timing synchronized with the main picture signal (1). Since the output signals from the analog switch (28) are separated into the luminance signal, red color difference signal and blue color difference signal, the latter two signals are modulated into a color signal in the chroma modulator (29). It is then summed up with the luminance signal by the adder (30), and an intended synthetic composite video signal (3) is obtained at the output terminal (31).
Since the conventional video signal synthesizer is thus composed, the main picture signal (1), which does not particularly require to be time-compressed is also demodulated and modulated. This caused a deterioration of picture quality of the main picture.
Further, the analog switch with three switching circuit systems is required for time-division multiplexing, which causes the cost to increase when the number of sub-pictures is increased.
Moreover, since the sync signal and color subcarrier signal of the sub-picture are generated on the basis of the sync signal and color subcarrier signal of the main picture, if the main picture signal is missing, then the reduced sub-picture signal is not delivered, either. Accordingly, when editing while simultaneously monitoring the main and sub-pictures, the editing job was disabled in such a case.
Incidentally, a similar video signal synthesizer of this kind, the apparatus in the Japanese Laid-open Patent Publication No. 56-48783 published on May 2, 1981 is known. In this apparatus, in order to prevent color deviation due to delay time difference between the luminance signal and the color difference signal of the reduced sub-picture signal, the delay time difference was corrected by adjusting the timing of writing the luminance signal and the color difference signal into the memory, or the timing of reading out thereof. As shown in FIG. 9 of the same publication it is similar to the prior art described above in that the main picture signal is separated into the luminance signal and the color difference signals which are then synthesized in the main/sub selector circuit.